1. Field of the Invention
This invention relates to apparatus providing protection in multiwire branch circuits of electric power distribution systems, and in particular, to circuit breakers providing protection from arcing faults and ground faults in such circuits.
2. Background Information
Branch circuits in electric power distribution systems often share a common neutral conductor to reduce the wiring required. Such multiwire branch circuits are often referred to as "home runs." Home runs are permitted as long as the two line conductors are energized by separate phases or by a center tapped single phase to avoid overloading the neutral conductor, and as long as all ungrounded conductors are disconnected simultaneously at the panel board where the branch circuits originate. This simultaneous disconnection of the ungrounded conductors can be accomplished with a two pole disconnect, two single pole circuit breakers with a handle tie, or a two pole circuit breaker.
Presently, such multiwire branch circuits are provided with short circuit and overcurrent protection by the tied single pole breakers or the two pole breaker. Only the two pole breaker can also provide ground fault protection by the addition of a common ground fault detector.
Recently, there has been an increased interest in providing protection from arc faults. Arc faults are intermittent high impedance faults which can be caused for instance by worn insulation, loose connections, broken conductors, and the like. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device which provides the short circuit and overcurrent protection. Various types of arc fault detectors have been proposed, but to my knowledge they have not been adapted to multiwire branches.
There is a need for a circuit breaker which can provide arc fault protection as well as short circuit and overcurrent, and ground fault protection for multiwire branch circuits.